...Why do people insult and neglect the human incarnation?

An ignorant person neglects Mahatma Gandhi seeing his cheap clothes. He cannot estimate the real value of Gandhi. He will give a lot of value to a cinema actor, who is in a colorful dress. Similarly, when the Lord comes in the human body, which follows all the rules of the nature, an ignorant person insults Him seeing the ordinary human body. He does not recognize the value of the internal form. This is told in the Gita (Avajaananti Mam Mudhaah). The Lord can make His body divine and above the rules of nature by His super power. But He does not do it because He does not like to violate the rules of nature, since He is the creator of those rules. One will not generally contradict His own rules and insult himself. An ordinary soul in the form of a yogi or a demon, frequently exhibits his body to be beyond the rules of nature because he is not insulted by violating them, since he is not the creator of the rules of the nature. By such petty miracles, an ignorant person believes the yogi or demon as the Lord. The Lord is the ruler of Yogis (Yogishwara).

Once a saint walked on the river but Shri Ramakrishna Paramahamsa, who was a human incarnation of the Lord, took a boat by paying one rupee.

The saint proudly told Shri Paramahamsa that he had obtained the power to walk on the water by rigorous sadhana that he had done for past thirty years. The saint asked Paramahamsa whether He had ever done such tedious sadhana in His life. Paramahamsa simply told him with a smile that the worth of his thirty-year sadhana was just one rupee and that He had never done such cheap sadhana. Arjuna prostrated at the feet of Lord Krishna before all the soldiers without egoism and jealousy. Krishna was a human being like Arjuna. Moreover Krishna was only the driver of the chariot where as Arjuna was the owner. However, Arjuna recognized the value of the internal form of Lord Krishna and therefore, Krishna revealed His inner form as the Vishwarupa to Arjuna. Therefore those people who get rid of jealousy and egoism can only give the value to the human incarnation.